1. Field of the Invention
The present invention relates to a casting method and apparatus in which a resin core is used.
2. Description of Related Art
In the die casting of a metal product, a core is used for forming an undercut portion and a vacant interior of the product. In a gravity die casting process, a sand core is usually used because the sand core can be easily removed from the product after casting because it collapses easily.
Recently, a resin core was proposed, for example, in Japanese Patent Publication No. HEI 6-91345, with a core made from thermoplastic resin that was removed from a product by heating the core and thereby melting the resin core. In order to prevent some of the melted resin from remaining in the product, a resin core removal method is proposed in Japanese Patent Application No. HEI 7-164299 where the resin core, having a substantially uniform wall thickness, is taken out of the cast metal product by drawing before it is melted so that the drawing force can be transmitted in the resin core and the entire portion of the resin core can be taken out of the cast metal product.
The above-described resin core, with a wall having a substantially uniform thickness, still has the following problems.
In a case where a cast metal product has a relatively thick portion (large thermal capacity portion), a portion of the resin core contacting the relatively thick portion of the cast metal product tends to be melted by receiving residual heat from the cast metal product. If this melting occurs, the accuracy of the shape and the dimensions of the portion of the cast metal product that contacts the melted portion of the core will be compromised.
A shrinkage recess or cavity may also be caused in a surface of a portion of the cast metal product that contacts the resin core when the portion is solidified after the other portions of the metal product.
Furthermore, the resin core may be destroyed in places if a load acting on a portion of the resin core from the molten metal, which flows into a molding cavity at a high speed and at a high pressure, exceeds the strength of the portion of the resin core.
The resin core may also be destroyed in places if a load acting on a portion of the resin core from a pressure pin exceeds the strength of the portion of the resin core.